Flue or “stack” gas contains combustion products that are produced when a fuel, such as coal, oil, natural gas, wood or the like is combusted in an industrial furnace, steam-generating boiler, or other large combustion device. These combustion products are released to the air through a flue-gas stack. The combustion products include carbon dioxide, nitrogen, and water vapor, and as well as a small amount of pollutants, including carbon monoxide, oxides of nitrogen and sulfur, and particulates. The levels of certain of these pollutants are subject to regulation and, as such, may require regular monitoring to assure compliance with environmental standards.
Flue gas is often monitored by a continuous emission monitoring system (“CEMS”). A complete CEMS typically includes: (i) sample transport and conditioning equipment, (ii) sample-gas analyzers, and (iii) data-acquisition and reporting equipment. Some CEMS are permanent installations; others include at least some portable elements, such as the sample transport and conditioning equipment.
After a sample of the flue gas is obtained from a stack, it is typically filtered and conditioned by the transport and conditioning equipment to remove moisture and particulates. Such conditioning must not alter the composition of the gas species being monitored in the sample. Filtering and conditioning is important because many sample-gas analyzers require a particulate-free, dry sample gas for reliable and accurate operation. The presence of condensable water vapor and particulates can result in a number of problems, including plugged conduits and flow components, obscured optical-sensing equipment, and can cause pumps, valves and flow meters to fail. Condensed water vapor is particularly problematic because certain components of interest, such as sulfur dioxide, readily dissolve in liquid water. In addition to resulting in an inaccurate (low) reading for sulfur dioxide in the gas stream being analyzed, the dissolved sulfur dioxide reacts with water to form sulfuric acid, which can corrode equipment.
The sample transport and conditioning equipment typically includes a filter, a sample probe, a sample line, and a gas conditioning system. FIG. 1 depicts an embodiment of sample transport and conditioning equipment in the prior art (hereinafter “portable sampling system”). The particular portable sampling system depicted is commercially available from PermaPure, LLC of Lakewood, N.J., as model GASS 35.
As depicted in FIG. 1, portable sampling system 100 includes filter 102, sample probe 104, heated transport line 112, and conditioning system 122, interconnected as shown.
Sample probe 104 obtains a sample of flue gas from the flue stack. The sample probe includes probe shaft (or “stinger”) 106 and probe body 108. Probe shaft 106, which is inserted into a flue stack to obtain a sample, comprises a metal, such as stainless steel or Hastelloy, suitable for exposure to high temperatures and the corrosive nature of flue gas. Probe body 108 provides heating, such as via heater 110, to prevent condensation from occurring, which would knock sulfur dioxide out of the gas sample. Filter 102, which is intended to filter out particulates from the flue gas sample, is fitted to the distal end of probe shaft 106 Filter 102 is typically a sintered metal or wire-mesh filter capable of filtering out particles as small as 10 microns.
Heated line 112 fluidically couples sample probe 104 to conditioning system 122. The heated line is typically about 2 to 3 meters (m) in length; this 2 to 3 meters affords a user an ability to place the conditioning system 122 on a supporting surface (e.g., the platform on which the user stands) while raising sample probe 104 as necessary to insert into the flue stack to obtain a sample. The heated line comprises insulated heating jacket 114 and tubing 116, the former enclosing the latter. Heating jacket 114 includes a heating element and thermal insulation. Tubing 116 typically comprises Polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP). Heated line 112 is controlled to a temperature in the range of about 120 to 180° C. This line is heated for the same reason that sample probe 104 is heated; that is, to prevent condensation.
Salient elements of conditioning system 122 include filtering elements 118, heater 120, two dryers 126A and 126B and two vacuum pumps 128A and 128B, interrelated as shown. Conditioning system 122 also includes a controller (not depicted) to control the operation of certain components of portable sampling system 100.
As previously indicated, sample transport and conditioning equipment, such as portable sampling system 100, must remove moisture from a flue-gas sample. This drying process utilizes a “purge” gas. The source and condition of the gas used as purge gas dictates, to some extent, the layout of the conditioning system. For example, different layouts will be required as a function of whether:                dry instrument air is available for use purge gas;        a portion of the flue-gas sample can be used as a purge gas;        flue gas cannot be used as purge gas, but relatively dry ambient air is available for such purpose and/or the required dew point of the conditioned flue-gas sample is not particularly low; or        flue gas cannot be used as purge gas and the ambient air is relatively wet and/or the required dew point of the conditioned flue-gas sample is very low.The use of two dryers and two vacuum pumps in conditioning system 122 enables portable sampling system 100 to be used in situations in which relatively wet ambient air is available as the purge gas and the required dew point of the conditioned flue-gas sample is quite low.        
Continuing with the discussion of FIG. 1, sample pump 124 draws flue gas sample 101 into sample probe 104, through heated line 112 and into conditioning system 122. Gas sample 101 is filtered via filtering elements 118, which may include a coalescing filter and an ammonia scrubber. Heater 120 heats the inlet line, filtering elements 118, and the first half (of the length) of dryer 126B. The heater includes a temperature sensor (not depicted), which transmits a signal to the system controller (not depicted). Based on the temperature signal, the controller adjusts the temperature as required to maintain a desired temperature in dryer 126B.
Dryers 126A and 126B utilize Nafion™, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, such as perfluorosulfonic acid (PFSA), tubing. Nafion™ is extremely selective at transferring moisture across its surface, such as from one flowing gas stream (typically on the inside of the Nafion™ tube) to another (on the outside of the tube). Dryer 126A, in conjunction with vacuum pump 128A, functions as a dry purge-gas generator. Dryer 126B, in conjunction with vacuum pump 1288, dries the flue-gas sample 101, using the dry purge gas generated by dryer 126A.
Ambient air 105 is drawn through line 130 into dryer 126A. The dried air exiting dryer 126A is split into two streams: recycle stream 109A and dry purge-gas stream 109B. Recycle stream 109A is drawn, under vacuum, across the “outside” of the Nafion™ tubing within dryer 126A as wet ambient air 105 moves through the inside of the Nafion™ tubing. The difference in the vapor pressure of water between wet ambient air 105 and recycle 109A drives moisture from the ambient air.
Dry purge gas stream 109B is drawn, under vacuum, across the “outside” of the Nafion™ tubing of dryer 126B as gas sample 101 moves through the inside of the Nafion™ tubing. The temperature at the inlet of dryer 126B is controlled to a temperature than is higher than the expected dew point. In some embodiments, the inlet temperature is about 80° C. Dried sample gas 103 exits dryer 126B and leaves conditioning system through line 132 for analysis in the gas analyzers (not depicted).
To remove moisture to desired levels, guidelines provide that the flow rate of purge gas 109B should be from one to three times the flow rate of the flue-gas sample 103 with a recommended vacuum level of 0.4 to 0.5 bar absolute.
Although effective for its intended purpose, there is a drawback to portable sampling system 100; namely, its weight. As a consequence of the location of a sample port on a flue-stack, an operator might be required to carry the portable sampling system up a considerable number of stairs. Due to the presence of two dryers, two vacuum pumps, filters, and heating equipment, conditioning system 122 weighs about 12.5 kilograms (kg). And heated line 112, which has an outer diameter of about 8 to 10 centimeters (cm) due to the presence of the heating element and a significant thickness of thermal insulation, weighs about 4.5 kg or more, depending on its length.
In light of the foregoing, there is a need for an improved method and apparatus to dry gas samples to very low dew points.